Research On Influence Of Pit Excavation On Adjacent Tunnels In Structural Soft Clay Foundation

With the rapid development of urban construction,the urban population is increasing,the scale of the city is expanding,and the buildings are becoming more and more dense.In that case,the subway has ignited a construction climax in all major cities in China,with the advantages of quickness and efficienfy.There are inevitably foundation pit construction near the built subway tunnel.However the coastal soft clay has a strong structure,large-scale underground space development will inevitably lead to structural changes in soil within a certain scale.When the soil structure is reduced to a certain extent,the mechanical properties of the sturctural soft clay will deteriorate rapidly,which easily leads to the catastrophic behaviors.The foundation pit construction is very likely to cause deformation in the adjacent subway tunnel.However,the existing tunnels are mostly assembled with shield segments,which are very sensitive to deformation,if the warning is not timely and remedial measures are taken,it is very likely to cause the tunnel to collapse.Therefore,this paper mainly focuses on the deterioration law of mechanical properties of coastal soft soil,the instability analysis of foundation pit in soft soil foundation and the influence of soft soil structure on the relationship between foundation pit tunnels.The previous centrifugal model tests on the influence of pit excavation on the adjacent subway tunnel were mainly based on the sand foundation,and the centrifugal model test study on the deformation of existing tunnels under the condition of foundation pit instability has not been found.In this paper,the centrifugal model test,laboratory unit test and numerical simulation were carried out.Firstly,centrifugal model test and numerical simulation was carried out to analyze preliminarily the influence of pit excavation on the adjacent subway tunnel in soft soil and progressive development process of shear band during foundation unstability.Then in order to analyze the influence of foundation pit excavation on adjacent subway tunnels in different structural soft soil foundations,laboratory unit tests were carried out to analyze the compressive properties,stress-strain characteristics and strength characteristics of coastal soft soil in Ningbo,and the degree of disturbance was used to evaluate different structure soils.Finally,the parameters of different structure soils obtained from laboratory tests were used for numerical simulation to analyze the influence of pit excavation on adjacent tunnels in soft soil foundation with different disturbance degrees,the conclusions are as follows:(1)When the soft soil foundation pit is in the stable stage during excavation,the retaining wall appears as a convex deformation,and the surface settlement on both sides appears as a groove-type settlement.The settlement point is located about half of the excavation depth away from the retaining wall,horizontal displacement and tensile deformation in the direction of the bottom of the pit will occur on the tunnels on both sides,and the closer the tunnel is to the foundation pit,the greater the deformation.(2)The progressive development of shear bands caused by kick-in-tpye deformation of retaining structure can be divided into three stages:strain concentration,strain localization generation and expansion,and shear band penetration,and the existence of the tunnel will change the development path of the shear band.(3)For the silty clay in Ningbo area,when the cement content of remolded soil is 2.0%,its compressive properties,stress-strain characteristics and strength characteristics are basically similar with the undisturbed soil.And in the range of 0%～2.0%cement content,with the increase of cement content in remolded soil,the compressibility index is linearly correlated with cement content,and the effective cohesion increases exponentially,while the effective internal friction angle does not change significantly,soil disturbance degree is negatively correlated with cement content.(4)With the increase of soil disturbance degree,the horizontal displacement of retaining wall,the ground settlement and the absolute displacement and maximun bending moment of tunnel increase sharply.When the soil disturbance degree reaches 11%and 36%,the horizontal displacement of retaining wall and ground settlement will exceed the control standard of the adjacent tunnel pit.And the absolute displacement of the tunnel increases fastest with the increase of soil disturbance degree,when the soil disturbance exceeds 38%and 39%,the maximum bending moment of the tunnel and its absolute displacement will also exceed the control standard respectively(5)Comparing the deformation of two sides of tunnel and pit,we can conclude that the horizontal displacement of retaining wall,ground settlement and tunnel displacement are affected by the distance between pit and tunnel during the process of pit excavation.When the tunnel is close to the foundation pit,the tunnel limits the displacement of the surrounding soil and bears a larger bending moment,and its displacement is also larger.The corresponding lateral surface settlement and the displacement of the retaining wall are correspondingly reduced.